<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      function fn(x = 20, y = +prompt("number?")) {
        console.log(x, y, x + y);
      }
      // fn(30, 50);
      // fn(, 50); // error
    </script>
  </body>
</html>
